1. Field of the Invention
The present invention relates generally to injection molding machines and, more particularly, to apparatus for production of injection molded articles from at least two supplies of plastic melt.
2. Description of the Related Art
Injection molding machines capable of producing two component molded articles are generally known from DE 36 20 175 C2, for example. In addition to ensuring a specified speed profile for closing and opening of the injection mold, the underlying task of the injection molding machine described in this patent is to produce injection molded articles from at least two plastic melts within a single machine cycle. Such molded articles might require, for example, melts having different material properties, different colors, or that produce transparent and opaque regions in the injection molded articles after solidification.
The injection molding machine described in DE 36 20 175 C2 includes the usual arrangement of a fixed mold mounting plate (attached to a machine frame) to accept a mold half, a counter-pressure plate also fixed on the machine frame at a spaced distance from the fixed mold mounting plate, these two stationary plates being connected by several (usually four) linear tie rods. The machine further includes a movable mold mounting plate that accepts a second mold half and is traversed on the tie rods by means of a clamping unit arranged between the counter-pressure plate and the moveable mold mounting plate. The clamping unit is of a known type, for example, a hydraulically operated piston-cylinder arrangement or a toggle mechanism that is actuated hydraulically.
This prior art patent provides a prismatic mold core support arranged between the two mold mounting plates and the mold halves mounted thereon. The mold core support is pivotable around an axis of rotation secured by two support rods that are aligned and movable parallel to the tie rods, so that the axis of rotation is perpendicular to the two support rods. The side surfaces of the mold core support are aligned parallel to its axis of rotation and serve to mount additional mold halves that can also be designed simply as cores. The positioning of the mold core support is adjustable by means of the pivoting movement so that always two of the side surfaces equipped with mold halves are exactly opposite a mating mold half on one of the mold mounting plates to form complete injection mold cavities with the mold halves opposite them on the mold mounting plates when the clamping unit is operated in the closing direction.
Supply of plastic melt to the injection molds described above, occurs by means of conventional plasticizing and injection units. More specifically, the injection mold cavity formed between the fixed mold mounting plate and core support is ordinarily supplied with plastic melt by a plasticizing and injection unit that is situated generally parallel to the tie rods on the side of the fixed mold mounting plate opposite the injection mold, and is positioned through an opening in the mold mounting plate to connect to a sprue channel running into the mold. The injection mold cavity formed between the moveable mold mounting plate and core support is supplied with plastic melt from a second plasticizing and injection unit by a sprue channel running through both the moveable mold mounting plate and the mold half mounted thereon. The sprue opening is situated in an outside surface of the moveable mold mounting plate aligned parallel to the longitudinal direction of the tie rods, so that the second plasticizing and injection unit is arranged essentially perpendicular to these rods and is only advanced to the sprue opening in the moveable mold mounting plate after complete closure of the injection mold.
If the prismatic core support has a cross-sectional surface perpendicular to its axis of rotation in the form of a regular, even-numbered polygon with the rotational axis being aligned essentially vertically, then in the closed state of the injection mold formed by the mold halves on the mold mounting plates and the core support, additional, adjustably arranged, mold mounting plates with corresponding mold halves and plasticizing and injection units can be advanced to the other side surfaces of the core support to mate with additional mold halves, in order to supply either additional plastic melts to produce the desired injection molded article or to produce more than one of the desired injection molded articles within a complete machine cycle. The minimal machine cycle then consists of the fact that a partial core is injected into one injection mold (preferably into the one formed by the mold halves on the fixed mold mounting plate) with a first plasticizing and injection unit, while at the same time a partial core produced in a preceding cycle step is completed to the desired injection molded article in a second injection mold (preferably in the one formed by the mold half abutting the moveable mold mounting plate) by a second plasticizing and injection unit with its plastic melt. The completed injection molded article is then ejected by an ejector at the desired location in or on the moveable mold mounting plate or in the core support, during the period from opening of the injection mold through pivoting the core support around its axis of rotation (ordinarily by an angle of 180.degree.) to position the mold halves held by the core support in front of the other mold mounting plate.
The two support rods for the mold core support are arranged parallel to the tie rods that connect the fixed mold mounting plate and the counter-pressure plate. The support rods are moveable, they secure the rotational axis of the core support, they are carried and guided by two openings in the moveable mold mounting plate, and they are interconnected by a cross-arm in the space between the moveable mold mounting plate and the counter-pressure plate. The cross-arm is connected to an actuator abutting the moveable mold mounting plate, for example, a hydraulically operated piston-cylinder arrangement or a suitably controlled stepping motor. The support rods ordinarily secure the rotational axis of the core support by means of bearings arranged on their ends facing the fixed mold mounting plate, in which the axis of rotation itself can be supported both in the essentially horizontal and in the essentially vertical position or also in an intermediate position as long as this support occurs perpendicular to the axis of the support rods.
The prior art design described above permits both a distinct pivoting movement of the core support (although the cited patent contains no details concerning the required mechanism) and a significant change in relative spacing between the core support and the moveable (and thus necessarily the fixed) mold mounting plate--assuming the spacing between the rotational axis of the core support and the mold mounting plates has a sufficient length. Based on the usual dimensioning of the support rods of the type shown in the prior art, only the use of a light and therefore small core support and correspondingly small mold halves is feasible; this limits the design to the production of injection molded articles of small dimension. However, a number of large dimensioned, large surface area parts, such as, bumpers, dashboards, sun roofs, etc. for vehicles are often injection molded from plastic melts, but such parts can only be produced from two or more plastic melts in a costly multistage injection molding processes, given the design limitations of the prior art machines discussed above.
The injection molds or mold halves and the associated core supports necessary to produce large, two or more melt, injection molded articles using the described prior art machine would require a very costly design of the support rods to carry the core supports, including the associated guides in the moveable mold mounting plate and a larger actuator, as well as a much more rigid designs for the moveable mold mounting plate, the machine's clamping unit, and the entire machine frame. These components would have to be made stronger (a) to accommodate the large-volume and/or large-surface mold halves, (b) to prevent the rods carrying the core supports from bending during the process of opening the injection molds, and/or (c) to prevent distortion of the bearings in the moveable mold mounting plate so that tight closure of the injection molds can be maintained.
Although not mentioned in the above-referenced patent, replacement of the core support is highly desirable to allow for flexible use of an injection molding machine of the present type, but the associated problems are significant. The sudden unloading of the support rods when removing the core support, or the sudden loading during installation of a core support, can lead to more or less dampened oscillations of the rods, resulting in very rapid material fatigue, hairline cracks, stress cracking and similar material damage. This damage creates a risk of sudden fracture of the rods, which could result in a significant hazard for the operating personnel. However, if the disadvantages of the prior art could be overcome, the applicability of the injection molding machine disclosed in DE 36 20 175 C2 would then, in principle, be attractive for production of large-volume and/or large-surface injection molded articles from at least two plastic melts.